1. Field of the Invention
The present invention relates to a lens for illumination for widening a range of transmission directions for light from a light source such as a light emitting diode, and to a lighting device using this lens for illumination. The present invention further relates to a surface light source including a plurality of lighting devices, and to a liquid-crystal display apparatus in which this surface light source is disposed behind a liquid-crystal panel to serve as a backlight.
2. Description of Related Art
In a conventional backlight of a large-sized liquid-crystal display apparatus, a number of cold cathode tubes are disposed immediately below a liquid-crystal panel, and these cold cathode tubes are used with other members such as a diffusing plate and a reflecting plate. In recent years, light emitting diodes have been used as light sources for backlights. Light emitting diodes have increased their efficiency recently, and are expected to serve as low-power light sources to replace fluorescent lamps. In the case where light emitting diodes are used as a light source in a liquid-crystal display apparatus, the power consumption of the apparatus can be reduced by controlling the light and dark states of the light emitting diodes according to an image to be displayed.
In a backlight of a liquid-crystal display apparatus using light emitting diodes as a light source, a large number of light emitting diodes are disposed therein instead of cold cathode tubes. The use of a large number of light emitting diodes allows the entire surface of the backlight to have uniform brightness, but the need for such a large number of light emitting diodes is an obstacle to cost reduction. Attempts to increase the output power of each light emitting diode to reduce the required number of light emitting diodes have been made. For example, Japanese Patent No. 3875247 has proposed a lens that is designed to provide a uniform surface light source with a reduced number of light emitting diodes.
In order to obtain a uniform surface light source with a reduced number of light emitting diodes, the area to be irradiated with the light emitted from each light emitting diode needs to be increased. That is, light emitted from each light emitting diode needs to be spread to obtain a wider range of transmission directions for light from the diode. For this purpose, in Japanese Patent No. 3875247, a lens having a circular shape in a plan view is disposed on a light emitting diode as a chip to control the directivity of the chip. The light exit surface of this lens, through which light exits the lens, has a shape such that a portion in the vicinity of the optical axis is a concave and a portion surrounding the concave is a convex extending continuously from the concave.
A light emitting diode as a chip emits light mostly in the front direction of the light emitting diode chip. In the lens disclosed in Japanese Patent No. 3875247, light that has been emitted in the front direction of the chip is refracted at the concave surface in the vicinity of the optical axis and diffused, as shown in FIG. 16A. As a result, the surface to be irradiated is illuminated to have a wide illuminance distribution with a reduced illuminance in the vicinity of the optical axis, as shown in FIG. 16B.
A light emitting diode does not emit light from a point but has a light emitting area of a certain size. Light emitted from the peripheral portion of the light emitting area travels along a path different from that of light emitted from the central portion of the light emitting area. In FIG. 16A, the paths of light rays emitted from the central portion of the light emitting area are indicated by solid lines, and the paths of light rays emitted from the peripheral portion of the light emitting area are indicated by dashed lines.
In order to obtain a wider illuminance distribution, it is conceived to reduce the curvature radius of the concave surface in the vicinity of the optical axis. If the curvature radius of the concave surface is reduced, the light rays emitted from the central portion of the light emitting area are incident on the concave surface at larger angles, and as a result, the light rays are refracted greatly to be directed outwardly, as indicated by solid lines in FIG. 17A. The light rays emitted from the peripheral portion of the light emitting area are, however, incident on the concave surface at excessively large angles locally as indicated by dashed lines in FIG. 17A and are totally reflected. Therefore, as shown in FIG. 17B, the illuminance on the surface to be irradiated drops locally, which forms a ring-shaped dark region thereon. As a result, a surface light source using the light emitting diodes has an uneven brightness. As described above, in the lens described in Japanese Patent No. 3875247, there is a limit to a widening of the range of transmission directions for light emitted from the light source due to the total reflection of the light emitted from the peripheral portion of the light emitting area.